With the rapid spread of Internet, the amount of data has been increased suddenly in the computer network market, and opportunities to add a storage system also have risen sharply in order to store the enormous data. A solution which can supply volumes timely to an abrupt demand for volumes centering on IDC (Internet Data Center), the so-called storage on-demand has been desired.
Each time the volume of a storage system is required, it has been necessary to perform a system adding operation. The system adding operation includes maintenance of power source equipment, ensuring of installing space, planning of an introduction schedule, supply and installation of a system, connection test, and delivery of the system to a user. A great deal of time and funds are required, so that the system cannot be easily added.
To reduce the number of addition times, a high-capacity storage system may be introduced at first. To do this, however, this increases the initial introduction cost which is a problem to a user who does not have enough funds. As a known example to such a problem, there is a function of IBM's “ESS capacity upgrade on-demand”.
In this function, in addition to the disk volume to be needed initially, an added disk volume (the capacity upgrade on-demand) is mounted for shipment, so that a user can use the added disk volume whenever necessary. A server processor additionally mounted is extended to enable the added disk volume to be used. The storage on-demand function has the merit of instantly adding and using the excessive added disk on customer demand. After introduction, however, a volume for the capacity upgrade on-demand must be purchased within one year.
The customer judges that a disk must be really added, not when the used disk is actually insufficient, but when the unused area of the usable disk is not enough. Naturally, there may be the case that disk addition is instructed when the total unused area is used and added data cannot be written. Since the system, which has already been in a state that data cannot be written thereinto, cannot be operated substantially, the case is late as the timing to add a disk. A system manager must always grasp the system state and sequentially add a disk to the area to be insufficient by the disk on-demand.
The disk on-demand solves the speed problem in that the adding operation can be efficient. In an aspect of a prior investment to be performed by predicting that storage data will be increased in the future, the problem how the wasteful prior investment is avoided still remains. Reduction of the added disk volume may be considered. In this case, however, addition opportunities are increased, which may easily exert some influence onto the system's long-time continuous stable operation. Charging opportunities by the disk on-demand are increased so as to provide a complex fee system.